1. Field of the Invention
The invention concerns devices and methods for controlling the relative power level of electromagnetic energy propagating along a path, and in particular concerns use of electrically controllable interference effects in one or more liquid crystal elements, to vary the extent to which incident light energy directed along a transmission path is allowed to pass through an output orifice, thus being transmitted through the device, versus light energy directed along other paths, thus being blocked by the device.
The invention is particularly useful in pixel displays, fiber optic data transmission lines and other applications that advantageously either switch incident light on and off or controllably attenuate the light. The device operates without polarizers and at low voltages.
The device relies on a particular physical layout whereby a controllable element, preferably a liquid crystal, is placed to intercept light oriented to impinge on an output orifice. In the preferred embodiment the controllable element comprises an electrically controllable random phase plate which creates phase interference in the forward direction, such that the amount of phase cancellation along a zero order path to the output can be electrically controlled.
2. Prior Art
It may be advantageous for various reasons to control whether or not light energy will be transmitted or blocked at a given point. In connection with displays, for example, it is useful to individually control transmission of light at each point or pixel in an array. In connection with light transmission devices such as glass fiber optical waveguides and the like, it may be desirable to switch light on and off for signaling or perhaps to control the transmission amplitude or light intensity over a control range. It is particularly advantageous if these sorts of controls can be effected electrically, that is by application of a voltage or current signal to alter light transmission conditions. Other control parameters besides electrical inputs may also be useful, such as mechanical or temperature related parameters.
One technique for electrical control of the properties of light is to pass the light through a liquid crystal. It is known to provide an array of liquid crystal elements defining pixels in a display device. Liquid crystals are also employed as elements in certain types of filters, e.g., in optical transmission situations such as glass fiber optical waveguides.
Liquid crystals produce or rely upon light polarization effects because they have distinct optical properties in mutually perpendicular axes; i.e., they are xe2x80x9cbirefringent.xe2x80x9d The two axes are known as the xe2x80x9cfastxe2x80x9d axis and the xe2x80x9cslowxe2x80x9d axis, or sometimes as the xe2x80x9cextraordinaryxe2x80x9d and the xe2x80x9cordinaryxe2x80x9d axes, ne and no. The liquid crystal material is typically positioned relative to an incident light beam such that these two axes, ne and no, define a plane normal to the propagation direction of incident light (the xe2x80x9cz axisxe2x80x9d).
The incident light also has distinct spatial components, namely components aligned wholly or partly to one or both of two mutually perpendicular polarization axes. There is an interaction between the polarization attributes of the incident light and the birefringence axes of the liquid crystal material.
This interaction is further affected by applying an electric field in the z axis direction (or potentially by using other effects such as temperature variation). Assuming an electro-optic effect, applying an electric field along the z-axis can alter the birefringence of the liquid crystal, specifically by changing the index of refraction of the liquid crystal material along the fast axis ne, and not along the slow axis no. As a result, the polarization component of the incident light that corresponds to ne may experience different optical changes compared to the component corresponding to no. In short, the polarization components can be affected differently by passing through the liquid crystal. A polarization filter or a beam splitter responsive to polarization may then be used to discriminate between the respective components, for example to turn on or off a pixel in a display or otherwise to operate light as a function of polarization.
Such polarization and birefringence aspects may be useful but not all of their characteristics are necessarily advantageous. For example, assuming randomly polarized incident light, a device with a polarization dependent transmission aspect inherently rejects 50% of the incident light energy. For this reason, electro-optic liquid crystal birefringence effects may be inconsistent with the need to preserve available light energy so as to maximize the brightness of a display. In some situations it may be possible to employ polarization diversity techniques to preserve the light energy. This could involve serially positioned components to split, realign and recombine orthogonal polarization components to reduce or eliminate rejection as a function of polarization. Such techniques entail expense, bulk and potential light energy losses reasons other than polarization rejection, such as elongation of the beam path.
Liquid crystal material conventionally is oriented to a reference direction on a substrate. In some processes this involves rubbing or abrading a surface of a substrate. At least for some thickness, molecules that are spaced inwardly from the surface tend to align with the elongations of abrasion, known as the xe2x80x9crubbing axis.xe2x80x9d
Typically display devices that use discrete liquid crystals to control pixel brightness rely on polarization effects. For common polarizer based displays, the backlighting must be polarized so that switched effects relying on polarization achieve reasonably good contrast. This results in at least a 50% loss of possible light energy. In most polarizer-based displays, two polarizers are involved. One polarizes the incident light and another discriminates on the basis of polarization aspects that are switched on or off at each pixel. This has led to efforts to develop single polarizer based devices or perhaps dual orthogonal discrimination elements. Ultimately, it would be advantageous to eliminate polarizers.
One device that does not use polarization is the polymer dispersed liquid crystal (PDLC). This device operates on a principle of scattering the light when in an xe2x80x9coffxe2x80x9d state and passing the light (i.e., becoming transparent) in the xe2x80x9conxe2x80x9d state. One disadvantage of a polymer dispersed liquid crystal (PDLC) device is that a polymer matrix surrounds the liquid crystal. The polymer matrix becomes part of an effective voltage divider, and reduces the voltage applied across the liquid crystal. The proportionate voltage reduction is determined by the effective capacitance of the polymer versus that of the liquid crystal. In some situations, to compensate for a considerable voltage drop across the polymer matrix, relatively large voltages must be applied across the device, e.g., on the order of 100V.
The switching operating principle of the PDLC is electrically to cause or prevent a mismatch in the index of refraction between the matrix and the liquid crystal. This changes the transmissivity/reflectivity characteristic of the boundary, making the light/dark appearance of the pixel controllable electrically.
It would be advantageous if a light handling technique could be developed that was free of the light energy rejection inefficiencies associated with polarization. However it would also be advantageous if the technique used low control voltages and modest power dissipation as typical of the electro-optic birefringent liquid crystal displays. Preferably, such a technique would reject as little incident light as possible, at least preserving more than the 50% level typical of a simple polarization dependent display. The technique should achieve a very high degree of contrast, using a low voltage, a good response time, and do so with a minimum requirement of additional components.
It is an object of the invention, among other objects, to employ the birefringence aspects of a controllable liquid crystal element as a phase interference element to produce switchable phase interference effects that control transmission of light along a transmission path, in a manner that is insensitive to polarization.
It is another object to achieve very high contrast between switched and unswitched conditions, using modest control voltages, while also permitting a continuous range of control when desired.
It is an object to optimize a device that meets all the foregoing objects, for applications including high density pixel displays in the visual band on one hand, and also fast switching glass fiber optical waveguide applications in the 1550 nm range.
These and other objects are accomplished by a controllable phase plate that produces a diffraction pattern having a portion, especially the zero order mode axial spot of the pattern, that is directed onto an output aperture such as a pinhole or an optical fiber end. By controlling the phase plate, an interference peak or null (or an intermediate level) is coupled into the output aperture. The phase plate preferably has a liquid crystal with controllable birefringence in small domains that have orthogonal director orientations. In a preferred arrangement the directors are randomized. This makes the device polarization insensitive. The device relies on having a propagation path for a light beam directed toward the output aperture, with the controllable phase interspersed along the path. Preferably, collimating lenses before and after the phase plate along the path produce a clear interference pattern focused in the area of the output aperture. Several variations are disclosed including an electrically controllable phase plate arrangement using liquid crystal controllably birefringent material prepared in a polarization insensitive manner in zones, or preferably by providing random director orientation in a plane.